Remove WalCompressionMethod in favor of pg_compress_algorithm

The same structure, with the same set of elements (for none, lz4, gzip
and zstd), exists in compression.h, so let's make use of the centralized
version instead of duplicating things.  Some of the variables used
previously for WalCompressionMethod are renamed to stick better with the
new structure and routine names.

WalCompressionMethod was leading to some confusion in walmethods.c, as
it was sometimes used to refer to some data unrelated to WAL.

Reported-by: Robert Haas
Author: Michael Paquier
Reviewed-by: Robert Haas, Georgios Kokolatos
Discussion: https://postgr.es/m/YlPQGNAAa04raObK@paquier.xyz

src/bin/pg_basebackup/walmethods.h

@@ -9,6 +9,7 @@
  *-------------------------------------------------------------------------
  */
 
+#include "common/compression.h"
 
 typedef void *Walfile;
 
@@ -19,15 +20,6 @@ typedef enum
 	CLOSE_NO_RENAME
 } WalCloseMethod;
 
-/* Types of compression supported */
-typedef enum
-{
-	COMPRESSION_GZIP,
-	COMPRESSION_LZ4,
-	COMPRESSION_ZSTD,
-	COMPRESSION_NONE
-} WalCompressionMethod;
-
 /*
  * A WalWriteMethod structure represents the different methods used
  * to write the streaming WAL as it's received.
@@ -68,7 +60,7 @@ struct WalWriteMethod
 	char	   *(*get_file_name) (const char *pathname, const char *temp_suffix);
 
 	/* Returns the compression method */
-	WalCompressionMethod (*compression_method) (void);
+	pg_compress_algorithm (*compression_algorithm) (void);
 
 	/*
 	 * Write count number of bytes to the file, and return the number of bytes
@@ -104,10 +96,10 @@ struct WalWriteMethod
  *						   not all those required for pg_receivewal)
  */
 WalWriteMethod *CreateWalDirectoryMethod(const char *basedir,
-										 WalCompressionMethod compression_method,
+										 pg_compress_algorithm compression_algo,
 										 int compression, bool sync);
 WalWriteMethod *CreateWalTarMethod(const char *tarbase,
-								   WalCompressionMethod compression_method,
+								   pg_compress_algorithm compression_algo,
 								   int compression, bool sync);
 
 /* Cleanup routines for previously-created methods */